In display devices and integrated circuits such as plasma display panels (“PDPs” hereinafter), field emission displays, liquid crystal display devices, fluorescent display devices, multilayer ceramic devices, and hybrid integrated circuits, a substrate is used in which electrodes and wires formed of Ag, Cu, or the like are formed on a substrate surface. In certain cases, these electrodes and wires are covered with insulating glass materials for protection. This is described below by taking a PDP as a representative example of display device.
Generally, a PDP has a configuration in which a pair of electrodes is arranged in an orderly manner respectively on opposing two glass substrates, and gases that mainly contain inert gases such as Ne and Xe are confined therebetween. Application of a voltage between the electrodes causes a discharge to occur in each tiny cell in the vicinity of the electrodes, causing the cell to emit light for display. For protection, the electrodes are covered with an insulating glass material called a dielectric layer.
For example, in a PDP of an AC type, the glass substrate serving as a front panel has transparent electrodes formed thereon, and metal electrodes having lower resistivity, such as Ag, Cu, and Al are formed on the transparent electrodes. These composite electrodes are covered with a dielectric layer, on which is formed a protective layer (MgO layer).
Generally, glass with a low softening point is used for the dielectric layer that covers the electrodes. The dielectric layer is formed by applying a glass powder-containing paste to cover the electrodes using a method such as a screen printing method or a die coating method, followed by baking.
The following lists some of the properties required for the glass composition forming the dielectric layer.
(1) Insulation to allow it to be formed on the electrodes.
(2) A thermal expansion coefficient that does not differ greatly from that of the substrate material, so that warping of the glass substrate and peeling or cracking of the dielectric layer can be prevented in a large-area panel.
(3) When used for the front panel, amorphous glass with high visible light transmissivity so that the light given off by the phosphors can be efficiently used as display light.
(4) A low softening point to match the heat resistance of the substrate glass.
An example of the glass substrate used for PDPs is soda lime glass that is produced by a float process and generally readily available as window sheet glass. Another example is high distortion point glass that has been developed for PDPs. Such glass generally has a heat resistance up to 600° C. and a thermal expansion coefficient of 75×10−7 to 85×10−7/° C.
Thus, for requirement (2), a thermal expansion coefficient of approximately 70×10−7 to 90×10−7/° C. is desirable. As to requirement (4), because the glass paste needs to be baked at temperatures no greater than 600° C., which is the distortion point of the glass substrate, the softening point needs to be at most 595° C., desirably at most about 590° C., so that the glass paste can soften sufficiently even when baked at or below 600° C.
At present, PbO—SiO2 glass whose main raw material is PbO is used mainly as the glass material satisfying these requirements.
However, with consideration given to recent environmental problems, there is a demand for a dielectric layer that is free from Pb. There is also a need to lower the dielectric constant of the glass material to reduce power consumption of the PDP. As such Pb-free glass, a Bi2O3—B2O3—ZnO—SiO2 glass material has been proposed that uses zinc borate as a main component and that includes Bi instead of Pb to attain a low softening point (for instance, see JP2001-139345A), for example. However, as in the case of the Pb materials, the Bi materials also have the problem of a high relative dielectric constant, roughly ranging from 9 to 13. Currently, there is a need for materials with a distinctly lower dielectric constant compared with these materials, specifically, materials with a relative dielectric constant of 7 or less, more desirably 6 or less.
In this connection, to achieve a low dielectric constant and a low softening point at the same time, there have been proposed materials with a relative dielectric constant of 7.0 or less using zinc borate glass that contains alkali metals instead of Pb (for instance, see JP9 (1997)-278482A, JP2000-313635A, and JP2002-274883A).
However, in the conventionally studied alkali zinc borate glass, the relative dielectric constant can be reduced to only 6.4 at best. Further, while it is possible to satisfy a low softening point, a suitable thermal expansion coefficient, and a low dielectric constant at the same time, it has been difficult to realize glass that has a high glass transition temperature (glass transition point) along with these properties.
If the glass simply is required to cover the electrodes, glass with a low softening point, a suitable thermal expansion coefficient, and a low dielectric constant would be sufficient. However, in PDPs, the glass layer is reheated to a temperature close to 500° C. in the post-processes of electrode covering, such as annealing of the MgO layer and the sealing process in which the front panel and the back panel are joined together. The softening point of the glass for the dielectric layer is slightly below 600° C., and as such application of heat about 500° C. does not soften the glass. However, when the applied heat greatly exceeds the glass transition temperature, the thermal expansion coefficient increases abruptly. In a large-area display in particular, this causes the dielectric layer to peel off from the substrate or cracks the dielectric layer, impairing insulation and reliability. According to studies made by the inventors of the present invention, a reheat treatment of about 500° C. requires the glass to have a glass transition temperature of desirably 465° C. or greater, more desirably 480° C. or greater. Display devices other than PDP, and circuit boards and the like are also at risk of the same kind of problem, when the high-temperature heat treatment is performed again after covering the electrodes and wires with the glass materials.
Studies by the inventors of the present invention have found that the boron content needs to be increased and the zinc content needs to be reduced to achieve a low dielectric constant in alkali zinc borate glass. While the glass transition temperature tends to be small in such composition range, there is completely no regard for the glass transition temperature in conventional glass for covering electrodes. Accordingly, materials with a high glass transition temperature along with a low softening point, a low dielectric constant, and a suitable thermal expansion coefficient have not been made yet.